Why do the leaves of the purple magnolia (Magnolia liliiflora) wither and drop?

A reader from Tianjin wrote in to ask: 4 years ago I bought 2 grafted purple magnolias (Magnolia liliiflora). They used to grow fairly well, but starting last year, yellow leaves appeared while they were overwintering in the flower cellar, and later this gradually developed into half of each leaf scorching and dropping off. After they came out of the cellar this May, the newly sprouted leaves soon turned half-withered and dry, and it was even worse from July to August. I originally thought it was caused by fungi, but applying fungicide had no effect. May I ask what is causing this? What rescue measures should be taken?

Answer: The withering and dropping of your purple magnolia leaves may be caused by the following aspects: First, they were put into the cellar too early, the temperature inside the cellar was too high, and ventilation was poor, which caused leaves to sprout during overwintering, and the new leaves grew poorly and became half-withered. Second, when they were taken out of the cellar, no adaptive hardening was done first, so they could not adapt all at once to the environment outside the cellar, which caused the new leaves to drop. Third, the summer environment was too hot, the light was too strong, and ventilation conditions were poor. Fourth, the potting soil had not been repotted and replaced for a long time, so the tips of the fleshy nutrient fibrous roots pressed against the pot wall, causing the root tips to be scorched during high temperature and dryness, which in turn caused the leaves to become half-withered; in addition, poor nutrition caused the leaves to turn yellow; compacted soil and excessive moisture caused root rot and withered leaves; and irrigation water that was not acidic enough and had a relatively high salt content could all cause the leaves to yellow and become half-withered.

The measures to be taken are as follows: First, before putting the plants into the cellar in winter, pick off all withered and old leaves and burn them to reduce the possibility of leaf spot disease. Second, the time for putting them into the cellar should wait until the temperature drops below 0°C, and the temperature inside the cellar during overwintering should be lowered, only enough to keep the potting soil from freezing; it is best if the potted plants do not sprout new leaves when they are taken out of the cellar. You may also consider burying them together with the pots in a wind-sheltered, sunny seedbed, and wrapping the aboveground parts well with rice straw or plastic film; they can tolerate at least -12°C, but be careful not to let the roots freeze. Third, from June to August, the potted plants can be moved to a cool, ventilated place that avoids the western sun, and water should often be sprayed on the plants and the surrounding environment to create a cool, moist, semi-shaded environment for them. Fourth, when they are moved out in spring, repot them and replace the soil, using a flowerpot slightly larger than the original one, pruning away the dead roots, cutting away 1/3 of the compacted old soil, and replanting them properly in loose, fertile, well-drained acidic potting soil. Fifth, during the growing season, pay attention to frequently loosening the soil and fertilizing, and add 0.1% ferrous sulfate powder to the irrigation water; this can promote vigorous plant growth, avoid physiological yellowing, and build a nutritional foundation for good flowering and abundant flowering the following year.